safety in pressure testing - hse

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Health and Safety Executive

Safety in pressure testingGuidance Note GS4

This is a free-to-download, web-friendly version of GS4 (Fourth edition, published 1998). This version has been adapted for online use from HSE’s current printed version.

You can buy the document at www.hsebooks.co.uk and most good bookshops.

ISBN 978 0 7176 1629 9 Price £6.00

This guidance gives advice on how pressure testing can be safely carried out by means of risk assessment, a safe system of work and suitable precautions.

In addition, its technical appendix gives advice on the design of protective barriers if there is a risk of pressure equipment rupture or detachment of a component while under test.

The guidance is aimed at employers, managers and supervisors of pressure testing teams/operators and self-employed persons.

HSE Books

Safety in pressure testing of 23


©Crown copyright 1998 First published 1977 Second edition 1992

ISBN 978 0 7176 1629 9

You may reuse this information (not including logos) free of charge in any format or medium, under the terms of the Open Government Licence. To view the licence visit www.nationalarchives.gov.uk/doc/open-government-licence/, write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email [email protected]. Some images and illustrations may not be owned by the Crown so cannot be reproduced without permission of the copyright owner. Enquiries should be sent to [email protected].

This guidance is issued by the Health and Safety Executive. Following the guidance is not compulsory and you are free to take other action. But if you do follow the guidance you will normally be doing enough to comply with the law. Health and safety inspectors seek to secure compliance with the law and may refer to this guidance as illustrating good practice.


ContentsIntroduction 4

Definitions 4

Risk assessment 5

Safe system of work 7

General precautions for pressure test 9

Pneumatic testing precautions 10

Hydraulic testing precautions 11

Underwater pneumatic testing 11

Further information from HSE books 11

Appendix Safety in pressure testing 12

References 20

References for table 1 in chronological order 20

Further information 23




Introduction 1 This guidance note gives advice to employers, managers and supervisors of pressure testing teams/operators and self-employed persons. It provides guidance on how pressure testing can be safely carried out by means of risk assessment, a safe system of work and suitable precautions. Its technical appendix gives advice on the design of protective barriers if there is a risk of pressure equipment rupture or detachment of a component while under test.

2 The appendix to this guidance note is based upon a research report commissioned by HSE and published as HSE Contract Research Report No CRR168 Pressure test safety. Its reference is given in ‘Further information’ at the end of this note. The report was prepared by G Saville and S M Richardson, Imperial College of Science, Technology and Medicine and B J Skilleme de Bristowe, BJS Research. Its 165 pages describe, in detail, a method for the quantification of the hazards associated with the conduct of pressure tests and with the sizing of protective barriers to prevent injury. It will be particularly useful to pressure testing sub-contractors and organisations specialising in the design and construction of barriers to contain blast and fragments.

Definitions3 In this document the term ‘pressure equipment’ means a pressure vessel, pipework, systems comprising one or more pressure vessels and associated pipework, and any other container of pressure, that need to be pressure tested.

4 Pressure equipment is normally pressure or leak tested after manufacture, repair or modification. It may also need to be periodically pressure tested as part of a routine inspection regime:

5 The term ‘pressure test’ in this document includes the following:

(a) Proof pressure test: this is carried out when the required thickness of all the pressure parts has not been accurately calculated or is in doubt. Its objective is to demonstrate the integrity of the pressure equipment. Proof pressure testing should only be carried out hydraulically and the pressure applied gradually until the specified test pressure is reached or until significant yielding of any part of the pressure equipment occurs. A method to determine significant yield is contained in British Standard 5500: 1997, amended 1998 Specification for unfired fusion-welded pressure vessels. (b) Standard pressure test: this test is used when the required thickness of all pressure parts has been calculated. Its objective is to prove the quality of materials used and the construction of the pressure equipment before it enters or re-enters service. This test is carried out at a specified pressure above the design pressure, typically 1.25 to 1.5 times the design pressure. (c) Leak test: this test may be carried out at a pressure not exceeding 10 % of the design pressure on pressure equipment which has not been subjected to a standard pressure test. It may also be performed at a higher pressure, not exceeding 110% of the design pressure, on pressure equipment which has satisfactorily passed the standard pressure test. (d) Functional test: this test is carried out using a suitable test medium at design pressure, or working pressure if this is lower, to check that the pressure equipment and its components function properly. It may include the actuation of moveable parts, such as the opening and closing of valves.



Hydraulic and pneumatic testing

6 Although pressure testing using a liquid as the pressurising medium (usually referred to as hydraulic testing) is not without risks, it is by far the safer method and should be used wherever practicable. Pressure testing using air, steam or gas as the pressurising medium (usually referred to as pneumatic testing) is potentially more dangerous because of the higher energy levels involved. For example, the energy released during a total failure of pressure equipment containing compressed air at pressures frequently used in pressure testing is more than 200 times the energy released by the same volume of water compressed to the same pressure.

7 Pneumatic testing should only be carried out when hydraulic testing is not practicable, for instance where the interior of the pressure equipment will be contaminated by the hydraulic test medium, or when the pressure equipment supports and/or foundations are not capable of supporting the weight of the equipment filled with the medium.

8 A pneumatic leak test within the limitations set out in paragraph 5(c) above can be used to find small but significant leaks, especially in equipment which will contain flammable gases and/or fluids.

Risk assessment 9 The first step to take before carrying out any pressure test is to perform a risk assessment of the operation. Risk assessment guides the judgement of the employer or the self-employed person as to the measures they need to take to carry out their legal obligations when pressure testing. It relies on the identification of all relevant hazards and dangers, and consists of an estimation of the risks arising from them with a view to their control or avoidance. For further guidance on risk assessment see the relevant HSE publications listed in ‘Further information’ at the end of this document.

10 As pressure testing is sometimes undertaken by specialist sub-contractors there is a need for co-operation and co-ordination between them and the pressure equipment manufacturer and/or owner to ensure that all risks are taken into account.

11 A risk assessment will indicate the extent and content of the safe system of work that needs to be in place before any pressure testing is carried out. A safe system of work is necessary to ensure the safety of testing personnel and other people in the vicinity, including the general public.

12 The assessment should also indicate whether or not pressure equipment needs to be placed behind a barrier when it is tested, bearing in mind that pressure equipment pneumatically tested will contain considerably more energy than the same equipment subject to hydraulic testing.

Hazards

13 The main hazard when pressure testing is the unintentional release of stored energy. In the case of pneumatic testing this can lead to a blast wave and missiles. However in the case of hydraulic testing, blast has little energy and it can be assumed that all expansion energy goes into missile energy.



14 The hazards presented by the compressibility, flammability and toxicity of the testing medium should also be considered.

Stored energy

15 Risk assessment of pressure testing activities should take account of the energy stored in the pressure equipment being tested and dangers arising when this stored energy is suddenly and unintentionally released, creating blast waves and missiles. The release of stored energy can be due to:

(a) rupture of pressure equipment due to brittle fracture; (b) rupture of pressure equipment due to ductile fracture; (c) detachment or removal of blanking plates and their clamps/attachment bolts, screwed plugs, isolation valves etc; (d) detachment of temporary welds on plugs, at pipe ends and nozzles.

16 Calculation of the stored compressed energy in the pressure equipment will indicate the extent of possible blast and missile formation if it ruptures or if components become detached under test pressure. Formulae for calculating stored energy in gas and liquid filled systems are shown in the appendix to this document. Further detailed information is available in HSE Contract Research Report No CRR168 Pressure test safety.

Blast and its effects

17 Blast and its effects on structures are briefly described in the appendix. The Research Report Pressure test safety reviews the physics of blast wave formation, discusses the response of structures to dynamic loads imposed by blast waves and missiles and gives some worked examples.

Missile formation

18 The appendix describes how the mass, size, shape and speed of each fragment can be determined. It also gives formulae for calculating the thickness of containment walls. Fuller explanations and worked examples are contained in Pressure test safety.

19 Possible modes of failure and hence fragment size should be the subject of discussions between the manager/supervisor of the pressure testing team /operators and the pressure equipment designer. In situations where pressure tests are carried out by sub-contractors they should obtain sufficient information from the designer and manufacturer for this assessment to be made.

Brittle fracture

20 The risk of rupture of the pressure equipment due to brittle fracture under the test conditions should be assessed at the design stage. This may affect the choice of materials of construction and the temperature at which tests will be carried out.



Safe system of work 21 A risk assessment should indicate the extent and content of the safe system of work that needs to be in place before any pressure testing is carried out.

Permit-to-work system

22 An important element of this safe system will be the written permit-to-work system. This should require:

(a) Recorded positive verification by a competent person that safe working conditions are in place before each pressure test commences. (b) Recorded authorisation by a competent person that the pressure equipment is isolated from pressure sources, fully de-pressurised and vented (or, if justified by a risk assessment, to any other safe level) before dismantling of the pressure equipment, its components or any remedial work takes place.

Training

23 The safe system of work should also require that all persons involved in pressure/leak testing are adequately and properly trained.

Written testing instructions and procedures

24 Written testing instructions should be available which include procedures for pressurising, de-pressurisation and venting. These should be drawn up by a competent person. Any changes in the instructions or procedures should be agreed in writing with the competent person. The instructions should define:

(a) the test pressure, the test duration and testing medium to be used; (b) where the testing medium supply line is to be attached to the pressure equipment; (c) the position and specification of safety valves and pressure gauges; (d) the position of isolation valves; and (e) the sequence for opening vent valves when more than one venting position is to be used.

25 If appropriate, an up-to-date piping and instrumentation drawing showing the position of all isolation valves, safety valves, non return valves, pressure gauges, testing medium supply points, vent valves and blanking plates should accompany these instructions.

26 Tables may also be needed as a checklist to show if valves are in the normally open or normally closed condition during pressurisation and de-pressurisation. This should ensure that when the pressure equipment is handed back for subsequent remedial or other work there is no residual pressure left in it. Any reference to the checking of valves should include remotely operated valves.

27 Depending upon the complexity of the pressure equipment under test, the system of work should include the following requirements:

(a) the dimensions of the designated test area, including minimum safe distances from the pressure equipment under test;



(b) the designated test area should be isolated as a no go area and testing should not start until all persons are in a safe place; (c) only those persons essential for the operation of the test equipment should be allowed in the vicinity of the test after pressurisation has begun; (d) checks to ensure that all test equipment used during the test is capable of withstanding the full test pressure; (d) the pressure should be applied gradually or increased by steps of approximately 10% until the required test pressure is reached. It may be necessary to record test pressure at suitable intervals; (e) pressure equipment at pressure should not be approached for close examination until a reasonable period of time has elapsed. The pressure at which it can be approached for close examination should be specified in the testing procedure. For the safety of testing personnel it may be necessary to consider remote viewing procedures rather than close physical examination; (f) unauthorised persons should not be allowed to approach the pressure equipment until it has been de-pressurised and vented; and (g) the pressure should be recorded after the equipment has been isolated from its pressure source and de-pressurisation/venting has been completed. This will indicate if there are any instances of re-pressurisation or failure to de-pressurise and vent before any work on the equipment takes place.

Venting

28 There should be sufficient venting positions to prevent the testing medium being trapped behind non-return valves, in dead legs or between isolation valves. Instruction should be provided on how the pressure equipment can be safely tested and vented in discrete sections, if this is the intended method of testing. Where the section under test terminates at a valve it should not be possible for pressure to build up inside an adjacent closed section if this valve leaks.

29 Before remedial or other work is to be carried out on the pressure equipment it should be vented and the pressure testing medium supply line disconnected. If it is unreasonable to disconnect the supply line then the isolation valve should be locked in the closed position.

Clamps and bolts

30 Clamps or bolts on bolted flanges must not be loosened or tightened while the pressure equipment is under pressure. The pressure equipment should be isolated from pressure sources, de-pressurised and vented before clamps or bolts are adjusted or removed.

31 Clamps should be fitted and tightened in accordance with the manufacturer’s recommended procedures. Flanged joints should be fitted with the appropriate bolts and gaskets and tightened in accordance with written procedures.

Test equipment

32 Pressure gauges should be fitted at or near each venting and pressurising medium supply point where they can easily be seen by the operator. Further pressure gauges should be incorporated to detect any residual pressure if there is a risk that pressurising medium can be trapped by check valves and similar equipment.



33 All pressure gauges used for pressure testing should be compared regularly with a calibrated gauge and records of the comparison kept. Any gauges used during the test should have a current certificate of calibration.

34 It may also be necessary to provide temperature gauges to check the testing medium temperature, if this is a relevant factor.

General precautions for pressure test 35 Care should be taken not to over-stress the pressure equipment during the test. Test pressures should be limited to ensure that the weakest part is not subjected to stresses greater than those specified in the appropriate standard or code.

36 Pressure equipment should not be subjected to any form of shock loading when under pressure.

37 To avoid brittle fracture the temperature of the medium entering the pressure equipment should not be lower than the test temperature stated in the testing procedure.

Evaluation and inspection prior to test

38 To minimise the risk of failure while under test, the pressure equipment should be evaluated and inspected before it is tested. The evaluation should include consideration of the design criteria, material specification and construction methods. The inspection may need to include visual and other non-destructive testing (NDT) methods to ensure that there are no significant defects which could lead to failure during the test.

39 Special attention should be given to the testing of pressure equipment which has been refurbished or repaired after a period of service. Such equipment may have been subjected to cyclic loading and/or corrosive environments. NDT may be necessary before the test to ensure that metal thickness meets specified design requirements and that there are no significant defects which may lead to failure during the test. Repair records should also be checked to ensure that any repairs were correctly carried out.

Protection against blast waves and missiles

40 Protection against blast waves and rocketing fragments can be provided by placing the pressure equipment behind a suitable barrier or in a properly designed pit. Formulae to calculate the thickness of barriers are shown in the appendix but more detailed information is given in HSE Contract Research Report Pressure test safety.



Safety valves

41 Safety valves of adequate size and marked with the set pressure should be installed in or close to the test supply line to prevent the test pressure being exceeded. It should not be possible to isolate these valves from the testing medium pressure. It is vital that safety valves are supplied and maintained in proper working order. They should be regularly stripped, cleaned and examined, and the set pressure checked before re-use.

Flexible pipes

42 Flexible pipes and their connections should be regularly examined to reduce the risk of them bursting or coming apart. Safety restraints should be attached so as to prevent flexible pipes flailing about in the event of them coming apart.

Personal protection

43 There is a risk of injury from particles of dirt and high velocity jets ejected during a hydraulic test at high pressure. A much greater risk exists during a pneumatic test. Personal protection including eye protection should be provided and used by persons working within the pressure test facility.

Pneumatic testing precautions 44 Where possible, steps should be taken to reduce to a minimum the internal volume of pressure equipment to be pneumatically tested e.g. by means of non-compressable material. This will reduce the amount of energy stored and so reduce the consequences of a rupture. It may be possible to isolate sections of the equipment and test them separately, followed by re-assembly and leak testing.

Large pressure systems

45 If the risk assessment of a pressure test indicates the pressure equipment should be placed behind barriers but it is too large or heavy to do so, then any persons in the test danger zone, including nearby buildings, public roads or open areas, should be kept clear until the test is over. It may be possible to carry out this test at night when it will cause the least disruption to the general public and normal production.

Flow regulation

46 The testing medium pressure should be controlled by reducing valves and flow control valves.

Test temperature

47 Local chilling during filling and emptying of the equipment when pneumatically testing can lead to brittle fracture. This risk should be minimised by avoiding sudden changes in flow rate across inlet and exhaust control valves or nozzles.



Hydraulic testing precautions 48 Pressure equipment being hydraulically tested should be totally filled with liquid and properly vented to exclude air pockets. In some cases the equipment may be so constructed that it cannot be fully flooded for a hydraulic test and the stored energy of the trapped air or gas creates a hazard. In these circumstances precautions suitable for pneumatic testing should be considered.

49 The effect on the test pressure of static head of hydraulic test medium should be considered and taken into account if necessary.

50 To avoid ice damage to the pressure equipment when hydraulically testing with water, the temperature during the test should not be less than 7ºC. Heating may need to be provided when there is a risk of the water freezing.

Underwater pneumatic testing 51 Underwater pneumatic testing is often used to combine a standard pressure test with a leak test for small, batch-produced pressure equipment.

52 The test tank should be properly designed and tested to withstand any sudden release of pressure safely. Deep immersion of pressure equipment in water will not provide enough protection if the test tank is incapable of withstanding the hydraulic shock of a pressure release.

53 Safeguards should be provided which ensure that:

(a) the equipment is completely immersed to its specified depth while the standard pressure test is being carried out, and (b) access into the designated danger zone is prevented.

54 Close observation of the equipment for leaks should only be permitted under the following conditions:

(a) when the leak test pressure is less than or equal to 10% of the design pressure, or (b) after a successful standard pressure test, when the leak test pressure does not exceed the design pressure.

55 It is recommended that the water temperature and depth be controlled.

Further information from HSE books Permits-to-work in the chemical industry INDG 98 (free leaflet)

Guidance on permit-to-work systems in the petroleum industry HSE Books 1997 ISBN 0717612813

Guide to risk assessment requirements: common provisions in health and safety law INDG 218 (single copies free; ISBN 0 7176 1211 2 for priced packs of five copies)



5 steps to risk assessment INDG 163 (single copies free; ISBN 0 7176 0904 9 for priced packs of ten copies)

Management of Health and Safety at Work Regulations 1992 Approved Code of Practice L21 HSE Books 1992 ISBN 0 7176 0412 8

HSE Contract Research Report Pressure test safety CRR168 HSE Books 1998 ISBN 0 717615421

The future availability and accuracy of the references listed in this publication cannot be guaranteed.

Appendix: Safety in pressure testing 1 Introduction

All types of equipment which operate at pressures above ambient need to be tested in order to check that they function correctly and, more importantly, that they are safe to use. All pressure testing has the potential for failure of the pressure system, and an assessment of both the risk and consequences of failure is essential.

A recent report, commissioned by HSE, has considered the procedures needed to carry out pressure testing safely. It includes comprehensive advice on calculation of stored energy, assessment of fragment size and velocity, assessment of blast and the shielding needed to contain both fragments and blast. The report is Saville G et al Pressure test safety. 1

This guidance is based on the above report and provides information on how to:

(a) assess the amount of energy stored in a pressurised system; (b) assess the potential for fragment generation and missiles; and (c) assess the thickness of materials needed to contain a pressure system failure.

An important general point is that the amount of energy stored in gas-filled systems is always considerably higher than in liquid-filled systems at the same pressure. The use of liquid as a pressurising medium is always the safer option.

2 Stored energy

Total system energy for a pressurised system will be the sum of the fluid expansion energy, the strain energy in the vessel and any chemically released energy. The release of chemical energy can normally be avoided by choosing a suitable test fluid and is not considered here. Strain energy is normally low by comparison with other energy sources, especially in gas-filled systems. This annex therefore concentrates on fluid expansion energy.

In estimating energy release it is normal to assume that the fluid expansion process is thermodynamically reversible and sufficiently rapid for heat transfer to the



surroundings to be negligible. The expansion is, therefore, isentropic and we can assume that the expansion energy, Ex, the change in internal energy of the closed system, ∆U, and the work done on the system, W, are of equal magnitude ie

Ex = - ∆U = - W

Gas-filled systemsFor gas-filled systems the following methods can be used for estimating internal energy:

(a) use of experimental data on fluid expansion; (b) use of empirical equations based on thermodynamic data; (c) assume the gas is an ideal or perfect gas; and/or (d) use the pressure-volume product.

These are listed in order of preference and are discussed in more detail below:

(a) For real gases, Table 2 in Pressure test safety provides a list of pure substances for which thermodynamic properties are available in tabular or chart form. A bibliography of the known sources of these data is also provided along with guidance on how it should be used. For convenience, these are included here as Table 1. The references will be found at the end of this annex.

Table 1 Pure substances for which the thermodynamic properties are available in tabular or chart form

Substance Reference No (see page 11)

AcetyleneAmmoniaArgonBenzenei - Butanen - Butane1 - ButaneCarbon dioxideCarbon monoxideCarbon tetrafluorideCyclopropanen - DecaneDichlorodifluoromethaneDichloromonofluoromethaneDichlorotetrafluoroethaneEthaneEthyleneHeliumn - Hexanen - Hydrogenp- HydrogenHydrogen sulphideMethaneMethyl chlorideMonochlorodifluoromethaneMonochlorotrifluoromethaneNeonNeopentaneNitrogen

(6)(11k)(19)(6)(17)(18a)(19)(2)(6)(17)(21 )(23)(30)(11l)(33)(1 )11f)(19)(1 )(11d)(17)(19)(5)(11j)(19)(2)(4 )(5)(6)(17)( 18b)( 19)(20)(2)(6)(13)(18c)(19)(17)(27)(1) (17)(17)(17)(1 )(6)(11 b)(17)(19)(29)(3)(6)(11 h)(17)(19)(35)(7)(10)(17)(32)(11 g)(19)(2)(15)(17)(18d)(19)(12)(17)(5)(1 )(6)(8)(11 a)(17)(19)(24)(28)(17)(17)(17)(16)(17)(33)(1 )(6)(9)( 17)(18e)(19)(23)



Fig 1 Fluid energy for nitrogen up to Fig 2 Fluid expansion energy for 200 bar nitrogen up to 4 kbar

Fig 3 Fluid expansion for various Fig 4 Fluid energy for various liquids liquids up to 1 kbar up to 10 kbar

Substance Reference No (see page 11)

Nitrous oxiden - NonaneOxygen n - PentanePerfluorocyclobutanePropanePropyleneSulphur dioxideTrichloromonofluromethaneTrichlorotrifluroethaneWater

(17)(5)(2)(17)(18f)(19)(23)(25)(31)(1 )(11 e)(19)(14)(1 )(6)(11 c)(17)(19)(5)(11 i)(17)(19)(17)(18g)(19)(17)(17)(2)(18h)(19)(22)(26)



(b) For many substances, thermodynamic data can be represented by equations of state which can be used, with other thermodynamic relationships, to calculate expansion energy. Reviews of those in current use can be found in Reid et al (1987)2 and Sandler (1994).3 Computer software is also available which will perform isothermal and isentropic expansion calculations for a wide range of substances and mixtures. Their proper use, however, requires a knowledge of thermodynamic and fluid properties. In the case of nitrogen, Saville et al have evaluated fluid expansion energy from the IUPAC equations of state (Angus et ai, 19794). Figures 1 and 2 are the resulting plots of fluid expansion energy against pressure. (c) We assume an ideal (perfect) gas for which

pV = nRT where p = system pressure (Pa) V = internal volume (m3) n = amount of fluid (mol) R = universal gas constant (8.314 J/K.mol) T = absolute temperature (ºK)

The energy released on expansion depends upon the thermodynamic path taken during expansion:

for isothermal expansion, Ex = pi Vi In (pf / pi)

for isentropic expansion, Ex = piVi -1

where pf = final pressure (Pa), Pi = initial pressure (Pa) Vi = initial volume (m3), = ratio of heat capacities cp/cv

In practice, no expansion is either completely reversible or entirely adiabatic and the real thermodynamic pathway will lie between the two. Isothermal expansion has been recommended by some authors as conservative. However, no real gas obeys the perfect gas equation and the effect of these imperfections can produce greater errors than the error involved in the assumptions about the expansion process.

(d) If all else fails, it is possible to use the pressure-volume product as a measure of expansion energy. Although simple, it does significantly underestimate expansion energy in most cases because it ignores the nature of the gas and the type of expansion process. If it must be used we recommend a safety factor of 1.5 for all pressures ≤ 50 MPa (500 bar) ie

Ex = npV where n = 1.5

Liquid filled systemsConservative values for the expansion energy of a liquid can be produced using the equation

∆U = 12V kt (pf

2-pi2)

where kt is the isothermal compressibility which can be found from pVT data. More accurate calculations are possible but are complex. Pressure test safety 1 gives further advice on how more accurate estimates of expansion energy of liquids can be produced. Where substances have had their pVT properties measured to very high pressures, it is possible to develop an equation of state to represent the pVT properties of the liquid. This has been done for water and a small number of other common substances. Saville et al have used these to produce the plots of fluid expansion energy against pressure shown here at Figures 3 and 4.

y

y-11 pf

pi

yy-1



For liquid-filled systems, it may be necessary to take account of strain energy, for example, when the pressurising liquid has low compressibility. Strain energy Es for a cylindrical vessel can be calculated from

3 (1- 2v) + 2K2(1+v) Es = K2-1

where E is the Young’s modulus of the vessel material (Pa) v is Poisson’s ratio for the vessel material K is the ratio of outside to internal diameter of the vessel

Examples Saville et al provide a number of examples of the use of the methods summarised here for estimating stored energy.

3 Fragmentation

When a pressure system fails, accident statistics suggest that every conceivable type of failure can occur, with the most common being the loss of plugs or closures and failure of the vessel itself, usually at welded seams. Although brittle failure of the vessel should not occur if pressure vessels are manufactured from the correct materials, it can be used to provide an upper band to fragment speed and is recommended for that reason.

Failure due to brittle fracture Experimental work suggests it is reasonable to assume that, for gas-filled systems, 40% of the expansion energy goes into fragment energy and 60% into blast. For liquid-filled systems, blast has little comparative energy and it should be assumed that all expansion energy goes into fragment energy. An upper band to fragment speed can be obtained by setting the total energy available (E) equal to the kinetic energy of the whole vessel ie

E = 12m V2 where m is the vessel mass (kg)

where V is initial fragment speed (m/s)

This speed can then be used for the main vessel fragments. Where additional mass is present, due to features such as flanges etc, the kinetic energy of the fragment will be the same as that of the area of vessel which it replaces. As a result of the increase in mass, therefore, the fragment velocity will be reduced. As a working procedure, we consider it reasonable to assume that fragments will be between 1% and 20% of the shell area and that identifiable features will be ejected intact eg end caps, closures, manhole covers, nozzles. The mass, size, shape and speed of each fragment will need to be determined.

Failure due to ductile fracture For ductile fracture, failure modes need to be identified by inspection by assuming fracture along lines of weakness such as welds. The force on the ejected fragment should be calculated from the pressure-area product and a period of acceleration assumed to take place until the fragment is clear by a distance equal to the diameter of the hole it leaves behind. The fragment speed (V) can be calculated from

2dpi A where d = diameter of hole left behind (m) V2 = mf A = area of ejected fragment (m2) mf = mass of fragment (kg)



Alternatively the fragment speed can be calculated on the assumption that it is given all the energy in the vessel, using

E = 12mV2

The lower velocity from these two methods should be used for calculating containment thicknesses.

For small fragments such as plugs and small closures, it should be assumed that d= twice the diameter of the hole left behind.

4 Containment of fragments

This advice on containment results from a review of data on missile penetration of targets. The advice produced is considered relevant for fragments with speeds of less than 500 m/s, a speed rarely exceeded by fragments produced by pressure vessel failures. Formulae for thickness of containment walls are generally empirical and those presented produce the best fit to existing experimental data. Formulae are for perforation, that is, the missile or fragment appears on the non-input side of the target but does not completely go through the target and continue on its way.

Mild steel The following formula is recommended:

t = 4.9 x 10-7 (M V2) 0.667

d

where t = thickness of containment for 50% perforation (m) M = mass of missile (kg) V = speed of missile (m/s) d = diameter of missile (m)

A safety factor of 1.25 is recommended in order to stop perforation of all missiles and, since the formula underestimates thickness for the thinnest shields, a minimum thickness of 3 mm is recommended.

Reinforced concrete The following formulae are recommended:

G(x/d) = 2.55 x 10-9 KNMV1.8

d 2.8

where G(x/d) = (x/2d)2 if G(x/d) 1(ie x/d 4.0) G(x/d) = (x/d)-1 if G(x/d) 1(ie x/d 4.0)

K = 1500 o 0.5

c

oc = ultimate compressive strength of concrete (Pa) N = 0.72 for flat-nosed missiles 0.84 for blunt-nosed missiles 1.00 for hemispherical-nosed missiles 1.14 for sharp-nosed missiles M = mass of missile (kg) V = speed of missile (m/s) d = diameter of missile (m) x = depth of penetration into an infinitely thick concrete block (m)

<<<

<



Perforation thickness is then calculated from

t/d = 3.19 (x/d) - 0.718 (x/d)2 for x/d ≤1.35 t/d = 1.32 + 1.24 (x/d) for x/d ≥1.35

The scabbing thickness, the thickness which first resists scabbing, is calculated from

s/d = 7.91 (x/d) - 5.06 (x/d)2 for x/d ≤0.65 s/d = 2.12 + 1.36 (x/d) for x/d ≥0.65

Use s for a plain reinforced wall. If a scab plate is used, the thickness of concrete can be reduced to t.

Polycarbonate Using typical values for bulk modulus (K) = 4 x 109 Pa Young’s modulus (E) = 2.4 x 109 Pa Yield strength (oy) = 100 x 106 Pa ultimate shear strength (Tu) = 65 x 106 Pa

we can use t = 1.61 M (V- a In(1 + bV )) bA b a

t = target thickness b = 0.25 (Kp)0.5 a = 2 u In (2z)

z = E / 1 + 2 E 0.5

oy oy

M = missile mass (kg) A = presented area of missile fragment (m2) V = speed of missile (m/s) p = density of target material (kg/m3)

[See Figure 3.11 in Pressure test safety]

Soil For compact missiles with a length to diameter ratio of 1 :1, the following formula is recommended:

x = 0.4 S M0.33 In (1 + 5.4 x 10-4 V2)

where x = soil penetration (m) S = soil parameter (0-5 for sand, 10 for average soil, 2.0 for soft soil) M = mass of missile (kg) V = speed of missile (m/s)

An alternative formula for missiles with length to diameter ratio > 10 is included in Pressure test safety.

Examples Again, Saville et al provide examples of how fragment size and velocity can be assessed and of how containment thicknesssess can be calculated for common containment materials.



5 Blast and its effect on structures

In comparison with the behaviour of missiles, our knowledge of blast waves is very limited. Most of the information available is for military explosives and for chemical explosions such as gas explosions. In order to assess the damage caused by a failed pressure vessel, it is assumed that the blast from a ruptured pressure vessel is the same as the blast caused by the detonation of the amount of TNT which will release the same energy ie 1 kg of TNT is equivalent to a stored energy of 4.5 MJ.

Although blast can largely be avoided by using liquid as the test medium rather than gas, the impact of missiles on protective walls has a blast-like effect and consideration of blast should still be made.

Saville et al review the physics of blast formation and how this has been used to develop relationships to predict blast effects from known data. For explosives and chemical explosions, this has been used to estimate safe distances from explosions and distances to which debris might be propelled. Factors which may need to be taken into account include:

(a) reflection of blast waves from unyielding surfaces; (b) energy yield magnification due to reflection of blast waves from the ground or from internal surfaces of a blast chamber; and (c) geometric effects due to local failure of a pressure vessel or due to the position of the vessel being non-central within a blast chamber.

Test chambersFor internal blasts within a test chamber, experimental work using charges of 5kg TNT has been carried out to produce pressure-time histories which have been approximated by a simple model. It is possible to use this approach to estimate the impulse load taken by the test chamber from the peak pressure and impulse received by the cubicle walls during the initial shocks. For totally enclosed chambers it is necessary to add on the pressure load and impulse due to the released gas. If the chamber is fully vented, for example with one wall completely open, then the addition of the pressure and impulse loads due to the released gas is not necessary.

Saville et al provide references for charts from which shock loading on chambers can be determined and describe a method for estimating the gas pressure loading for unvented chambers. Additionally they provide a simple method for estimating the blast-like effect of missile impact. An example is given of how this approach can be used to calculate peak pressures and impulse loads.

Structural response to blastBlast from an explosion damages a structure by causing it to deform. Damage is most likely when the period of the blast impulse is close to the natural vibration period of the structure. If the blast impulse period is long compared to the natural period, the structure can be considered to be loaded quasi-statically. In this case the displacement of the structure is related solely to the peak force produced by the blast and the stiffness of the structure. If the blast impulse period is short when compared to the natural period, the blast load will be over before the structure has moved. This is known as impulsive loading. Where the blast impulse period is close to the natural period, deflection will be between the static and impulsive cases and it will be necessary to solve equations of motion numerically to obtain the response of the structure.

It is clearly important that damage assessment is not based on the peak blast pressure alone; if loading is impulsive rather than quasi-static then severe over-estimation of damage could result.



Saville et al show how the blast response of structures made from flat plates, including steel and concrete, can be represented as a pressure impulse diagram so that the pressure-impulse combinations which result in structural deformation can be predicted. Example calculations are given for representative structures.

References 1 Saville G et al Pressure test safety CRR168 HSE Books 1998 ISBN 0717615421

2 Reid R C et al The properties of gases and liquids 4th ed McGraw-Hill 1987

3 Sandler S I (Editor) Models for thermodynamic and phase equilibrium calculations Dekker 1994

4 Angus S et al International thermodynamic tables of the fluid state - nitrogen Pergamon 1979

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2 Hilsenrath J et al Circular 564 American National Bureau of Standards (ANBS) 1955

3 Benzler H and Koch A V Chem. Ing. Tech. 19552771

4 Cramer F Chem. Ing. Tech. 1955 27484

5 Sage B Hand Lacey W N API Research project 37 (monograph) American Petroleum Institute 1955

6 Din F Thermodynamic functions of gases vols 1-3 Butterworth 1956-61

7 Mann D Band Stewart R B Technical note 8 ANBS 1959

8 Edminster W C Applied hydrocarbon thermodynamics Gulf Publishing Company 1961

9 Strobridge T R Technical note 129 ANBS 1962

10 Mann D B Technical note 154 ANBS 1962

11 Canjar L N et al Hydrocarbon Processing and Petroleum Refiner (a) 1962 41 (9) 291; (b) (10) 149; (c) (11) 203; (d) (12) 115; (e) 1963 42 (1) 129; (f) (8) 127; (g) 1964 43 (6) 177; (h) 1965 44 (9) 219; (i) (10) 137; 0) (10) 141; (k) (11) 293; (I) (11) 297

12 Roder H M et al Monograph 94 ANBS 1963



13 Hurst J G and Stewart R B Technical note 202 ANBS 1963

14 Harrison R Hand Douslin D R Perfluorocyclobutane: the thermodynamic properties of the real gas US Department of the Interior 1964

15 Kubin R F and Presley L L SP 3002 Nat. Aero. Space Admin. 1964

16 McCarty R D and Stewart R B Advances in thermodynamic properties at extreme temperatures and pressures ASME 1965 84

17 Am. Soc. of Heating, Refrig. and Air Cond. Engineers Thermodynamic properties of refrigerants 1969

18 Canjar L N et al Hydrocarbon Processing and Petroleum Refiner 1966 45 (a) (1) 135; (b) (1) 139; (c) (2) 158; (d) (2) 161; (e) (3) 137; (f) (3) 143; (g) (4) 161; (h) (4) 165

19 Canjar L N and Manning F S Thermodynamic properties and reduced correlations for gases Gulf Publishing Company 1967

20 Vukalovich M P and Altunin V V Thermodynamic properties of carbon dioxide Collets 1968

21 Gasman A L et al National standards reference data series NBS 27 1969

22 Keenan J H et al Steam tables - international edition in SI units Wiley 1969

23 Vasserman A A and Rabinovich V A Thermophysical properties of liquid air and its properties Israel Programme for Scientific Translations 1970

24 Zagoruchenko V A and Zhuravelev A M Thermophysical properties of gaseous and liquid methane US Department of Commerce 1970

25 Weber L A J Res. Nat. Bur. Standards 1970 74A 93

26 UK steam tables in SI units Arnold 1970

27 Lin DC K et al J Chem Eng Data 1971 16 416

28 Starling K E Hydrocarbon processing 1971 50 (4) 139

29 Starling K E and Kwok Y C Hydrocarbon processing 1971 50 (4) 140

30 Angus Sand Armstrong B International thermodynamic tables of the fluid state - argon 1971 Butterworth 1972

31 Roder H M and Weber L A Oxygen technology survey vol 1: Thermophysical properties’ NASA 1972

32 McCarty R D Thermophysical properties of helium-4 Technical note 622 ANBS 1972

33 Thermodynamic properties of benzene Item number 73009 Engineering Sciences Data Unit 1973

34 Dawson P P and McKetta J J J .Chem. Eng. Data 1973 18 76



35 Angus S et al International thermodynamic tables of the fluid state - ethylene 1972 Butterworth 1974

Further informationFor information about health and safety, or to report inconsistencies or inaccuracies in this guidance, visit www.hse.gov.uk/. You can view HSE guidance online and order priced publications from the website. HSE priced publications are also available from bookshops.

Published by HSE 04/11 of 23


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